Automotive Wire Size Calculator

This is a simple calculator for determining approximate wire gage/size based on the length of the wire (in Feet) and the current (in Amps) in general Automotive applications. This can be helpful if doing your own repairs or adding custom wiring to make sure each circuit is not overloaded. When doing automotive wiring it is important to use the proper wire in the proper place, and this also goes for the operating temps of wire plastic casings (insulation). You don't want to run common cheap wire in a hot engine compartment, so make sure that you know the temperature range of your application and the temperature rating of the wire you are using. We have found that the "big box" automotive parts stores sell GPT wire (General Purpose Wire) that we do not recommend for use in high temperature applications, such as under hood. You are unlikely to find GXL/TXL wire in a parts store. While shopping for wire, you may see wire with specs like -

GXL, and TXL wires are oil, gas, acid and generally chemical proof. One other note about wire type is Teflon and Tefzel are generally very good wire but expensive. And always check the ratings. We have seen Teflon wires with a temp rating of 105C(221F) which is below the typical cross-linked as indicated above. If you are going to get Teflon or Tefzel wire, it will be silver plated conductors and expensive. Typical temperature range for Teflon is -60C to 200C (-76F to 392F) and Tefzel is -70C to 150C (-94 to 302F). Sometime you may see wire called PTFE, PTF for Teflon as well. A few sources exist for Teflon wires, aircraft supply, surplus sometimes, even a few racing places are starting to sell it. We don't because it's overkill for most of our customers and there are not enough customers to warrant getting into selling it.

The voltage drop value will vary depending on your application. We prefer using the 2% mark as a conservative value and to ensure delivering full potential to any needed electrical source. From what it looks like in many automotive applications a 5% or more voltage drop is more common.

As a general rule, the smaller the diameter of the wire the higher the resistance and thus lower current capability over a given length. It is almost always OK to use a larger gauge wire. When in doubt about the load go up a gauge. Other factors can affect the capacity of wire besides the length, and this includes if it is in a hot environment, duration of load, stranded vs. solid wire, wire plating, etc. Some aircraft Teflon wires have high strand count and are silver plated. These wires have higher capacity then typical copper stranded wires. Check out Wikipedia for more info on wires (AWG, Brown and Sharpe), gauges, and the math. Approximate Metric Wire Sizes Equivalents (with some rounding) are also include in the table as diameter/area and are in milimeters/square milimieters [mm/mm2], and again use larger size if in question.

NOTES: A) This is for a ONE WAY wire with the ground assumed to be the car chassis. If 2 wires needed to complete the circuit calculate for the total length of wire used for both legs. To use the calculator, enter the Maximum Current in Amps of the circuit, and the Length of the Wire, and operating Voltage. Typically most automotive systems run at 13.8V, but you can select 6, 12, 13.8 (Default) or 24 volts. If the wire gauge is OK to use it will have a 'Check Mark' on the same line. The 'Max Length' indicates the maximum length of wire of that gauge that can be use at the given values you entered.B): We believe the table to be an accurate general guide for MOST applications, however you agree to use at YOUR OWN RISK. By using our wire calculator, you agree to hold us harmless and you understand that this calculator is merely a guide and help, but not an absolute authority, therefore we recommend you have a professional or an engineering company perform the calculations in your application. C This calculator ONLY applies to chassis wiring (individual wires exposed to open air for cooling) and not power transmission wiring (a large wire harness is an example of power transmission wiring). Large wire bundles (harnesses) are more complex and this calculator does not address those complexities. In power transmission wiring, internal wires are not exposed to air, therefore the heat dissipation is reduced and each situation requires its own analysis.